Polyolefin resins, such as polypropylene, for use in nonwovens are difficult to produce and pose additional challenges compared to polyolefin resins suitable for films. This is because the material and processing requirements for production of fibers are much more stringent than for producing films. For the production of fibers, the residence time available for structure formation is typically much shorter and flow characteristics are more demanding on the material's physical and rheological properties. Also, the local strain rate and shear/extensional rate are much greater in fiber production than other processes and, for spinning very fine fibers, small defects, slight inconsistencies, or phase incompatibilities in the melt are not acceptable for a commercially viable process.
Generally, smaller fiber diameters will enable softer nonwovens. Softer nonwovens are desirable as they are gentler to the skin, feel right to the touch and help provide a more garment-like aesthetic for diapers, wipes and other like products.
Another desirable attribute sought in the fibers comprising nonwovens, besides softness, is abrasion resistance. Abrasion resistance is also important since it ensures that both the fibers and the nonwovens possess sufficient mechanical integrity during use so as to not fall apart and produce undesirable fuzz or lose aesthetics.
Nonwovens that are capable of high extensibility at relatively low force are also desired. These can be used to provide sustained fit in products, such as diapers and the like, and facilitate the use of various mechanical post-treatments. Typically, it has been found that having both a smaller fiber diameter and an easy to extend fiber are difficult to achieve. This is because, when the fiber diameter is reduced, it is commonly done by increasing the spinning speed or draw ratio during spinning which decreases extensibility of the fiber in post mechanical treatment due to increased polymer orientation.
More recently, there has also been a growing need in the industry for nonwovens that can also exhibit significant extensibility or cold-drawability when used in disposable products. Indeed, for absorbent articles such as diapers and catamenials, solid-state activation processes have become an integral part of the fabrication of many chassis components. Such processes can provide important functional benefits: improved softness or hand that increase a nonwoven's comfort and feel; added loft, texture or aperturing that enhance visual appearance, alter transport properties or desirably modify mechanical properties. In such processes, however, the nonwoven needs to remain intact after being stretched at a high strain rate.
One way the art has used to address these issues is to blend various polymeric resins. For example, U.S. Pat. No. 6,476,135 describes blends of as-polymerized, high melt flow rate (MFR) propylene homopolymer (250 to 550 g/10 min) and a random copolymer of propylene and ethylene and/or a C4-C10 α-olefin which are suitable for production of extensible fibers. Nonwoven fabrics that are formed with fibers prepared using propylene impact copolymer compositions are disclosed in published PCT applications WO 01/64979 A1 and WO 01/64979 A1. U.S. Pat. No. 5,804,286 describes a fiber, nonwoven and multilayered fabric comprising various polymers such as isotactic polypropylene, polyethylene and a block or grafted polyolefin copolymer or terpolymer which is at least partially miscible with the polypropylene and the polyethylene. Multilayered structures are also claimed in U.S. Pat. No. 6,506,698 and WO 00/28122 U.S. Pat. No. 5,616,412 discloses fine denier filaments having a high elongation at break that comprise a blend of polypropylene and polystyrene where the blends are made by forming an intimate blend using a twin screw extruder and then spinning the blend. Published PCT application WO 01/73174 A1 describes a method of making a fabric comprising a plurality of fibers, the fibers comprising a least one polypropylene polymer and at least one ethylene polymer.
Another way the art addressed these issues is through bicomponent fibers. For example, published PCT application WO 01/30563 describes an elastic laminate employing a nonwoven layer that includes bicomponent fibers having a sheath-core structure. The sheath contains an ethylene-propylene random copolymer that contains about 7 mol % to about 15 mol % of ethylene comonomer randomly distributed in the polymer backbone. U.S. Pat. No. 6,417,122 discloses multicomponent fibers comprising at least two polymer components arranged in structured domains, where each component comprises a multi-polymer blend of at least two different polyolefins with the higher melting phase being either the dominant continuous phase or the non-continuous phase.
The use of random copolymers or stereoisomers of polypropylene in fibers has also been described. For example a spunbonded fabric that consists of fibers with a diameter of from about 5 to about 40 microns, that are made of polypropylene copolymer blends with high ethylene content (>10 wt %) for at least one of the components is disclosed in U.S. Pat. No. 6,235,664. U.S. Pat. No. 6,080,818 discloses a fiber, thread or yarn that includes a polymer blend of a predominantly atactic flexible polyolefin with an isotactic polypropylene, a method for preparing such a fiber, thread or yarn and nonwovens products prepared therefrom.
As noted above, there exists an unmet need for highly extensible nonwovens with fibers that can be made from commercially available thermoplastic resins without the need for high cost specialty polymers or complex manufacturing processes. It is well known that, as spinning speeds increase, molecular orientation increases, stress to further deform the fiber increases and fiber elongation decreases. This is ideal for producing low denier fibers with high strength and low deformability. However, producing fine fibers with high extensibility at an affordable cost remains a very significant challenge.
There is also a need for a polypropylene material suitable for use in a fibrous nonwoven which is readily extensible (especially at high strain rates) and has improved abrasion resistance.